Method for etch processing with end point detection thereof

ABSTRACT

A method for performing process end point detection in a semiconductor substrate processing system by monitoring for an increase in a flow of backside gas above a predetermined limit.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/219,888, filed Aug. 14, 2002, now U.S. Pat. No. 6,837,965 which ishereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to semiconductor substrateprocessing systems. More specifically, the present invention relates toa method and apparatus for performing etch process end point detectionin a semiconductor substrate processing system.

2. Description of the Related Art

During the manufacture of semiconductor devices, a deep trench plasmaetch provides non-mechanical separation simultaneously for all chips ona substrate (also referred to herein as a wafer). Being a highlyproductive process, a deep trench plasma etch process has found wide usein semiconductor wafer processing systems.

The term “deep trench plasma etch” is broadly used to refer to processesused to manufacture devices on silicon and non-silicon substratescomprising a processing step that plasma etches through a bulk ofmaterial of the substrate.

A requirement in such processes is a prompt termination of etchingimmediately after the first through, or clear, opening has beendeveloped in the substrate. Therefore, reliable and accurate end pointdetection is critical during deep trench plasma etch. However, duringdeep trench plasma etch, the conventional end point detectors do notoperate reliably.

There are generally two classes of background art systems for end pointdetection used during plasma etching, both require at least one viewingport in the etch chamber. The first class of systems includes laserinterferometric detectors. These detectors focus a laser on the materialto be etched and monitor the phase of the light reflected from thematerial. As the material is etched (removed), the phase of thereflected light changes in proportion with the depth of the etch. Inthis manner, the detector monitors the etch depth and can cause theetching process to stop upon achieving a predetermined depth. To measureminute phase changes, the equipment must be accurately calibrated, andsuch equipment requires repeated recalibration. Also, as line widthsbecome narrower, maintaining the laser focus upon a bottom of a trenchis becoming difficult.

The second class of the systems includes optical emission spectrometry(OES) detectors. These detectors comprise a data acquisition system anda plasma optical emission receiver and detect a change in intensity ofone or several wavelengths of the plasma optical emission related to anetched or underlying layer. Sensitivity of these detectors reduces witheither complexity of spectrums or intensity of the plasma as thespectral lines of interest become obscured by background spectrum.

To identify the occurrence of a deep trench plasma etch extendingthrough the wafer, the change in the spectrum that occurs when backsidegas escapes into the chamber through the trench is detected. Forexample, during the etch process, a backside gas (e.g., helium) isprovided to the interstitial spaces between the wafer and the pedestalto promote heat transfer. As such, the gas leaks into the chamber fromthe edges of the wafer throughout the process. Therefore, the plasmaalways contains some amount of the backside gas. Upon the trench etchingthrough the substrate, a small amount of additional backside gas escapesthrough the trench into the chamber. This additional gas alters theemission spectrum of the plasma. The end point detector can monitor thisspectral change and stop etching upon its detection. However, thespectral change is so small that it might be missed until the etchingprocess forms a substantial opening. Because the spectral change issmall, any plasma non-uniformity may mask the signal.

If the end point is missed during deep trench plasma etch, there is arisk of plasma damage to the substrate pedestal and a risk ofcontamination of the pedestal by sub-products of the etching process andcontamination of an etch chamber by a material from the pedestal. Inmany plasma chambers, a portion of the pedestal supporting a substrateis an electrostatic chuck that can be damaged even by accidentalexposure to plasma or by the contaminants arising from exposure to theplasma.

Therefore, a need exists in the art for reliable end point detectionduring deep trench plasma etch.

SUMMARY OF THE INVENTION

The present invention is a method for determining the end point of aplasma process that uses an increase in a value of a flow of backsidegas. In one embodiment of the invention, an increase in the flow ofbackside gas is detected after an opening in a substrate has been formedby a plasma etching process. The invention provides reliable and timelydetection of the process end point.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe had by reference to the embodiments thereof which are illustrated inthe appended drawings.

It is to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic diagram of a semiconductor wafer processing systemthat can be used for deep trench plasma etch having an end pointdetection system in accordance with one embodiment of the presentinvention;

FIG. 2 is a flow diagram of a method of end point detection inaccordance with one embodiment of the present invention;

FIG. 3 is a schematic diagram of an end point module in accordance withone embodiment of the present invention; and

FIG. 4 is a timing diagram of flow and pressure of backside gas and anoutput signal of the end point module of FIG. 3 during a deep trenchplasma etch process.

DETAILED DESCRIPTION

The present invention is a method and apparatus for determining the endpoint of deep trench plasma etch process.

FIG. 1 is a schematic diagram of one embodiment of a semiconductor waferprocessing system 100 generally comprising an etch processing chamber104, an end point detector 130, and a computerized controller 102. Theend point detector 130 comprises a data acquisition system (DAS) 146, anend point module 136, and an interface 138. The DAS 146 is similar to aDAS of a conventional OES detector used with a plasma optical emissionreceiver.

The controller 102 comprises a central processing unit (CPU) 150, amemory 152, and support circuit 154. The controller 102 is coupled tovarious components of the chamber 104 to facilitate control of the etchprocess. The controller 102 regulates and monitors processing in thechamber 104 via interfaces that can be broadly described as analog,digital, wired, wireless, optical, and fiber-optic interfaces. Tofacilitate control of the chamber as described above, the CPU 150 may beone of any form of general purpose computer processor that can be usedin an industrial setting for controlling various chambers andsub-processors. The memory 152 is coupled to the CPU 150. The memory152, or computer-readable medium, may be one or more of readilyavailable memory such as random access memory (RAM), read only memory(ROM), floppy disk, hard disk, or any other form of digital storage,local or remote. The support circuits 154 are coupled to the CPU 150 forsupporting the processor in a conventional manner. These circuitsinclude cache, power supplies, clock circuits, input/output circuitryand subsystems, and the like.

An etching process is generally stored in the memory 152 as a softwareroutine. The software routine may also be stored and/or executed by asecond CPU (not shown) that is remotely located from the hardware beingcontrolled by the CPU 150. The software routine, when executed by theCPU 150, transforms the general purpose computer into a specific purposecomputer (controller) 102 that controls the chamber operation such thatthe deep trench plasma etching process is performed. Although theprocess of the present invention is discussed as being implemented as asoftware routine, some of the method steps that are disclosed thereinmay be performed in hardware as well as by the software controller. Assuch, the invention may be implemented in software as executed upon acomputer system, in hardware as an application specific integratedcircuit or other type of hardware implementation, or a combination ofsoftware and hardware.

The chamber 104 may be used for plasma-enhanced processes. Suchprocesses include, but are not limited to: deep trench plasma etch,inductively coupled plasma (ICP) etch, pre-clean sputter etch, plasmaenhanced chemical vapor deposition (PECVD), and related processes usedto manufacture semiconductor devices, very large scale integration(VLSI) devices, computer and optical media, opto-electronics,micro-mechanical systems (MEMS), and the like.

The chamber 104 comprises an upper portion 106 (e.g., a roof or a dome),an antenna 108 coupled to plasma generator 112 (e.g., an RF powersource), and a lower portion 110. The upper and lower portions aresealed to one another and, together, define a vacuum chamber. The lowerportion 110 comprises a substrate pedestal 116, gas panel 118 to supplyprocess gas, and vacuum pump 120 for providing a vacuum and evacuationof the used gases and volatile sub-products.

The pedestal 116 generally is coupled to a bias source 122, a backsidegas system 135, a chuck 126 for retaining the substrate 114, andtemperature controller 128 for establishing a process temperature thesubstrate 114. The backside gas system 135 comprises a gas source 124, apressure monitor 132, a mass flow controller 134, and backside gasplumbing lines 140, 142, 144. The chuck 126 can be either anelectrostatic chuck or a mechanical chuck mounted atop of the pedestal116. The chuck 126 maintains the substrate 114 at a stationary locationduring substrate processing.

The gas source 124 contains a pressurized, high purity backside gas,which generally is an inert gas such as helium, argon, and the like or acombination thereof. The pressure monitor 132 measures the pressure ofthe backside gas in a space between a non-processed surface (i.e., the“backside”) of the substrate 114 and a support surface of the chuck 126.In one embodiment of a chuck, the support surface contains grooves thatare coupled to the backside gas line 140. The grooves form a pattern tofacilitate uniform gas distribution beneath the substrate.

Backside gas serves as a heat transfer agent between the substrate 114and the chuck 126. During deep trench plasma etching, the chuck 126 iscooled using coolant supplied by the temperature controller 128 to atemperature lower than that of the substrate 114 and, therefore, thebackside gas assists in cooling the substrate 114.

During processing of the substrate 114, the controller 102 adjusts theflow of the backside gas using the monitor 136 to achieve and maintain anominal pressure of the gas. The pressure typically is controlled in arange of 4-16 Torr and, in particular, in a range of 8-12 Torr. Therange of 8-12 Torr generally facilitates the most efficient heattransfer between the substrate and the chuck. The controller 102 uses aninterface 148 to the controller 134 to perform adjustments of the flowto achieve and maintain the nominal level of the pressure of thebackside gas.

During the etch process, a nominal level of the flow of the backsidegas, corresponding to the nominal gas pressure, is established in thesystem 100, and the flow rises when a punch through of the substrate 114occurs. The flow controller 134 generates a voltage that is proportionalto the gas flow through the flow controlled 134. This voltage is coupledto the end point detection system 130. When the voltage representing theflow exceeds a preset limit, the output signal of the end point module136 exceeds a level defined for the end point event. The limit for theflow of the backside gas generally is established experimentally duringcharacterization tests of deep trench plasma etch prior to processingthe product substrates. The output signal of the module 136 is deliveredby the interface 138 to the DAS 146.

In the present invention, the output signal of the module 136 is scaledto be a substitute for the optical end point signal otherwise producedby a plasma optical emission sensor. The module 136 operates as avoltage-to-optical signal converter. The signal from the flow controller134 is converted by the module 136 into an optical signal. When thesignal received from the module 136 becomes equal or greater than apredetermined limit, the DAS 146 defines the end point for the etchprocess and submits this information to the controller 102. In response,the controller 102 terminates the plasma, and deep trench plasma etch ofthe substrate 114 is stopped.

In one embodiment of the present invention, the DAS 146 and controller102 utilize the same end point detection software that conventionallyused to control a plasma. To reduce the electrical noise and increaseelectrical immunity between the module 136, the chamber 104, and the DAS146, in one embodiment, the interface 138 is an optical cable, and, morespecifically, a fiber-optic cable. The optical cable may be aconventional cable for optical emission monitoring used to couple theDAS to a window in the etch processing chamber. In one embodiment of theinvention, the cable is removed from the window and connected to theoutput of the module 136. The DAS 146 receives the optical signal andconverts the signal to an electrical format for further processing bythe DAS 146 and the controller 102. As such, the module 136, the massflow controller 134, and the DAS 146 do not have a direct electricalconnection to one another, i.e., the module 136 operates as anopto-coupler.

FIG. 2 is a flow diagram for a method of end point detection inaccordance with one embodiment of the present invention shown as asequence 200. The sequence 200 comprises steps 201-205. At step 201,settings for a nominal flow of the backside gas and for a flow at theend point are stored in the memory 152. At step 202, a flow the backsidegas begins and a controlled level of the backside gas pressure isreached and maintained. At step 203, a deep trench plasma etch processis performed upon the substrate 114. Step 204 is a decision step whereinthe controller 102 defines whether the end point has been detected. Anend point is detected (i.e., an end point event) when the module 136produces a sufficiently large optical signal in response to an increasein the voltage representing the backside gas flow. If the end pointevent does not occur, the process 200 continues etching the substrate.At step 205, upon an end point event occurring, the deep trench plasmaetch process is terminated in the chamber 104.

FIG. 3 is a schematic diagram of the end point module 136 in accordancewith one embodiment of the present invention (component pin numbers andpower connections are not shown). The module 136 comprises a two-stageamplifier 300 having a first stage 302, a second stage 304, an opticaloutput load 306, and an optical adapter 308. An input signal from theflow controller 134 is provided through an electrical connector 301. Thefirst stage 302 comprises “Input Scaling” potentiometer 310, “InputOffset” potentiometer 312, “Input Gain” potentiometer 314, the inputfiltering capacitors 316, 318, and an output filtering capacitor 320.The stages 302 and 304 are generally operational amplifiers,specifically, operational amplifiers model Super 741. The nominal valuesof the potentiometers 310, 312, and 314 are about 20 kΩ, and the nominalvalues of the capacitors 316, 318 and 320 are about 0.1 μF. The secondstage 304 comprises the gain setting resistors 322 and 324 having thenominal values of about 2 kΩ and 18 kΩ, respectively.

The optical output load 306 comprises a high intensity light emittingdiode (LED) 326, a LED 328, and a current limiting resistor 330 having avalue of about 540 Ω. The LED 326 emits a broad spectral range “PureWhite” light, and the LED 328 emits a narrow spectral range “Ultra Blue”light. The wavelengths are selected to emulate the expected opticalspectrum that is monitored by the DAS. For detecting the helium producedspectra, the LEDs produce at least blue light. Other detection spectramay be used to make the module 136 compatible with an existing DAS. BothLED are selected for the most efficient conversion of an electricaloutput signal from the second stage 304 into an optical signal. Althoughthe depicted embodiment has two LEDs, other embodiments may use one LEDor more than two.

The optical signal is coupled by the adapter 308 into an optical cableand transmitted to an optical input of the DAS 146. The adapter 308performs coupling of the light emitted by the LED 326 and the LED 328into the optical cable. The adapter 308 is generally an opticalcondenser. The optical cable is generally a fiber-optic cable. Intensityof light emitted by the LED 326 and the LED 328 depends highlynon-linearly from an output voltage of the stage 304, sharply increasingwith the output voltage. Voltage gain of the stage 302 is adjusted usingthe potentiometers 310, 312, and 314. Voltage gain of the stage 302 isset by the resistors 322 and 324. Combined gain of the stages 302 and304 is adjusted in a manner that provides a peak of the emitted light(also referred to as an optical output) at about a level of thepredetermined limit for a flow of the backside gas at the end point(i.e., at punch trough), thus increasing sensitivity of end pointdetection by the DAS 146.

FIG. 4 depicts an exemplary timing diagram representing a method fordetermining an end point during a deep trench plasma etch process inaccordance with the invention. Solid and dashed lines are used in thegraphs of FIG. 4 to indicate the controlled and uncontrolled values,respectively. Specifically, shown in FIG. 4 is a graph 400 depicting avalue of a flow 402 of the backside gas (axis 404) versus time (axis406), a graph 420 depicting an electrical output signal 422 from thesecond stage 304 (axis 424) versus time (axis 426), and a graph 440depicting an intensity of an optical output 442 (axis 444) of the module136 versus time (axis 446).

As described with respect to FIG. 4, the flow of a backside gas beginsduring period 460. Next, as the plasma is ignited and maintained, theflow is stabilized by the moment T1 at a nominal level 408 during period462. During period 464 starting at the moment T2, the backside gas flowincreases as the plasma begins punching through the substrate. At themoment T3, the flow reaches a level 410 corresponding to an end pointduring deep trench plasma etch, and the flow and the plasma areterminated. Lastly, the flow of the backside gas stays terminated duringperiod 466 until deep trench plasma etch of the next substrate begins.The signal 422 is proportional to the signal 402. The signal 422 reachesa level 428 and a level 430 at moments T1 and T3, respectively. Thelevels 428 and 430 of the signal 422 correspond to the levels 408 and410 of the flow 402 at the moments T1 and T3, respectively. The opticaloutput 442 stays at a low level 448 during period 462. However, asdiscussed above, specific adjustments of an electrical gain in themodule 136 result in the optical output 442 having a sharp peak 450 atthe moment T3 when a flow of the backside gas reaches the level 410 thatset for a flow at the end point.

In one embodiment, the invention is used in the deep trench silicon etchchamber (known as a Decoupled Plasma Source (DPS) chamber) with a plasmaprocessing system model Centura 5200, manufactured and sold by AppliedMaterials, Inc. of Santa Clara, Calif., USA. Other process chambers andsystems that require sensitive end point detection will also find theinvention useful.

While foregoing is directed to the preferred embodiment of the presentinvention, other and further embodiments of the invention may be devisedwithout departing from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method of processing a substrate on a support pedestal, comprising:etching a substrate disposed on a support pedestal; monitoring a valueof a flow of a backside gas to a space between the substrate and thesupport pedestal; continuing to etch until at least one hole is punchedthrough the substrate, thereby allowing the backside gas to leak therethrough; and detecting an end point of the process performed on thesubstrate when the value of a flow of the backside gas increases above apredetermined limit.
 2. The method of claim 1, wherein the process is adeep trench plasma etch process.
 3. The method of claim 1, wherein thevalue of a flow of a backside gas to a space between the substrate andthe support pedestal is monitored by a controlled gain amplifier thatproduces a signal corresponding to the value of the flow of the backsidegas, said signal indicative of the end point of the process when saidvalue in creases above the predetermined limit.
 4. The method of claim3, wherein the controlled gain amplifier comprises a first and a secondamplifying stages, an input filter, and a filter between an output ofthe first stage and an input of the second stage.
 5. The method of claim4, wherein the controlled gain amplifier further comprises an opticaladapter.
 6. The method of claim 5, wherein the second amplifying stagehas an output load comprising at least one light emitting diode and acurrent limiting resistor.
 7. The method of claim 6, wherein at leastone light emitting diode emits a high intensity light.
 8. The method ofclaim 6, wherein at least one light emitting diode emits a broadspectral range “Pure White” light.
 9. The method of claim 6, wherein atleast one light emitting diode emits a narrow spectral range “UltraBlue” light.
 10. The method of claim 6, wherein the light emitted by atleast one light emitting diode is coupled by the optical adapter into afirst end of an optical cable, and wherein a second end of said opticalcable is coupled to an input for an end point signal of a dataacquisition system detecting the end point of the process.
 11. Themethod of claim 1, wherein detecting the end point of the processfurther comprises: providing an optical output corresponding to the flowof the backside gas; and monitoring the optical output using an opticalemission sensor.
 12. The method of claim 11, wherein providing theoptical output comprises: emitting light from at least one LED toemulate the expected optical spectrum of the process monitored by theoptical emission sensor.
 13. A method of processing a substrate on asupport pedestal, comprising: performing a process on a substratedisposed on a substrate support pedestal resulting in leakage of abackside gas through the substrate due to etching a hole in it, thebackside gas provided to a space defined between the substrate and thesupport pedestal; and ending the process in response to a changedetected in a flow of the backside gas due to this leakage.
 14. Themethod of claim 13, wherein the step of ending further comprises:detecting an increase in flow above a predetermined limit.
 15. Themethod of claim 13, wherein the process is an etch process.
 16. Themethod of claim 15, wherein the process is a deep trench plasma etchprocess.
 17. The method of claim 13, wherein the step of ending furthercomprises: monitoring a value of flow of the backside gas by acontrolled gain amplifier; and producing a signal from the controlledgain amplifier corresponding to the value of the flow of the backsidegas, the signal indicative of the end point of the process when thevalue increases above a predetermined limit.
 18. The method of claim 13,wherein the step of ending the process further comprises: providing anoptical signal indicative of the flow of the backside gas.
 19. Themethod of claim 18, wherein the step of providing the optical signalfurther comprises: emitting a light having a wavelength selected toemulate an optical spectrum of the backside gas.
 20. The method of claim19, wherein the step of providing the optical signal further comprises:emitting a broad spectral range “Pure White” light.
 21. The method ofclaim 18, wherein the step of providing the optical signal furthercomprises: emitting a narrow spectral range “Ultra Blue” light.
 22. Themethod of claim 18, wherein the step of ending the process furthercomprises: comparing the optical signal to a predefined value.
 23. Amethod of processing a substrate on a support pedestal, comprising:providing a predefined flow of backside gas to a space defined between asubstrate and a substrate support; etching the substrate to cause aleakage of the backside gas through the substrate; and determining anendpoint in response to a change in the backside gas flow caused by theleakage.
 24. The method of claim 23, wherein the step of determining theendpoint further comprises; detecting an increase in backside gas flow.25. The method of claim 23, wherein the step of detecting furthercomprises; comparing the flow to a predetermined limit.
 26. The methodof claim 23, wherein the step of etching further comprises: plasmaetching a deep trench into the substrate.
 27. The method of claim 23,wherein the step of determining further comprises: monitoring the flowof backside gas by a controlled gain amplifier; and producing a signalfrom the controlled gain amplifier corresponding to the flow of thebackside gas, the signal indicative of the end point of the process whenthe value increases above a predetermined limit.
 28. The method of claim23, wherein the step of determining the endpoint further comprises:providing an optical signal indicative of the flow of the backside gas.29. The method of claim 28, wherein the step of providing the opticalsignal further comprises: emitting at least one of a broad spectrumrange “Pure White” light or a narrow spectral range “Ultra Blue” light.